Applicants claim, under 35 U.S.C. xc2xa7119, the benefit of priority of the filing date of Aug. 7, 1997 of a German patent application Serial Number 197 34 136.5, filed on the aforementioned date, the entire contents of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a scanning unit for an optical position measuring device. The position measuring device furnishes not only periodic incremental signals but also at least one reference pulse signal at a defined relative position of a scale and a scanning unit that is movable relative to the scale. To that end, at least one reference marking field is disposed on the scale, integrated into the incremental graduation track.
2. Description of the Related Art
Known optical position measuring devices, of the kind used for instance in machine tools to detect the relative position of the workpiece and the tool, as a rule generate not only incremental signals with respect to the relative offset but also so-called reference pulse signals. By way of the reference pulse signals, at one or more defined relative positions of the parts movable relative to one another, an exact absolute reference can be established for the position measurement. To generate the reference pulse signals, reference marking fields are disposed at one or more positions on the scale of the respective position measuring device. The scanning unit of such a position measuring device offers the opportunity, at the relative position of the scale and scanning unit that is to be detected, of generating a corresponding reference pulse signal that is suitably processed in the downstream evaluation device.
With respect to the disposition of reference marking fields on the scale, various possibilities now exist. For instance, it is known from U.S. Pat. No. 4,263,506 to dispose the reference marking fields on the scale laterally adjacent to the incremental graduation track. A problematic aspect of such an arrangement, however, is that in the event of a possible twisting of the scale and the scanning unit about an axis perpendicular to the plane of the scale, or scanning plane, the exact, positionally correct association of the reference pulse signal with a defined period of the incremental signals is no longer assured.
It is also possible for one or more reference marking fields to be integrated directly into the incremental graduation track, as has been proposed for instance by U.S. Pat. No. 3,985,448. The aforementioned disadvantages that result particularly in the event of twisting of the scale and the scanning unit, can be avoided if the reference marking fields are disposed in the incremental graduation track. The optical scanning of an incremental graduation track into a which a reference marking field is also integrated is done in the aforementioned reference via a scanning unit that on the one hand has a suitably embodied scanner plate with corresponding scanning graduations and on the other a suitable detector system. One problematic aspect among others in scanning and incremental graduation track embodied in this way and having at least one integrated reference marking field is that in the region of the reference marking field, only a sharply impaired incremental signal is available.
To circumvent this problem would require a very long reference marking field on the scale in the measuring direction. This in turn means a bulky scanning unit.
Along with generating the incremental signals using a scanning unit which has not only the appropriate detector elements but also a scanner plate, scanning units are known that have a so-called structured detector system. On the scanner, active detector regions are provided adjacent one another in the measuring direction on a semiconductor substrate, and they each generate certain signal components of the scanning signals. In such scanning units, a single component thus takes on the combined function of a scanning graduation and a detector element. In this respect, see European Patent Disclosure EP 518 620 A1. However, this reference does not disclose how, with such a detector system, a reference marking field integrated directly into the incremental graduation track can be scanned, and in which the aforementioned problems with regard to the reduced incremental graduation track in the region of the reference marking fields are sharply reduced.
From German Patent Disclosure DE 195 12 258, a structured detector system is known that is used to generate a reference pulse signal. However, once again the reference markings are disposed laterally adjacent to the incremental graduation. Accordingly, the aforementioned problems again occur if the scale and scanning unit twist about an axis or perpendicular to the plane of the scale, or scanning plane. The embodiment of the detector system proposed in this reference is not suited for scanning a reference marking that is integrated directly into the incremental graduation track.
It is an object and advantage of the present application to disclose a scanning unit of compact structure for an optical position measuring device, which in scanning a scale generates not only incremental signals but also a reference pulse signal at least one defined position. The influences of error on generating the reference pulse signal that result from the aforementioned twisting of the scale and scanning unit are intended to be minimized, as is the vulnerability to any possible contamination of the scale. In the generation of the reference pulse signal, the least possible interfering influence on the incremental signal is also desirable. Finally, furthermore, the phase relationship of the reference pulse signal relative to the incremental signals should be preserved even if the scale should possibly tip about an axis in the plane of the scale.
This object is attained by a scanning unit as defined by a scanning unit for an optical position measuring device that includes a detector system that has a first reference pulse signal detector region disposed along a measuring direction and a second reference pulse signal detector region disposed along the measuring direction. The relative disposition of the reference pulse signal detector regions in the measuring direction is selected as a function of the structuring of a reference marking field on a scale for generating a reference pulse signal. An incremental signal detector region is disposed along the measuring direction and between the first and second reference pulse signal detector regions to generate at least one incremental signal.
The embodiment according to the present invention of the detector system in the scanning unit now assures the desired insensitivity in the event of possible twisting of the scale and scanning unit about an axis perpendicular to the measurement plane. The location of the reference pulse signal generated relative to the incremental signals does not change even in such a case.
It is furthermore assured that even in the region of the reference marking field, signal components from the detector system are available for generating at least incremental signal. In addition, the incremental signal is only slightly affected. In this way, appropriate precision for the position determination is assured.
In an advantageous feature of the present invention, a scanning unit of extremely compact structure can be attained because of the embodiment according to the present invention of the detector system.
In the aforementioned embodiment of the scanning unit of the present invention, a so-called single-field scanning can also be assured. This means that all the phase-displaced signal components that contribute to generating the various optical scanning signals originate in a single graduation period of the scale graduation. This assures increased insensitivity to contamination, for instance on the scale. The quality of the various scanning signals is affected uniformly in each case by any possible contamination.
In addition, however, it is understood also to be possible, along with the single-field scanning arrangement mentioned, to embody alternative scanning arrangements in accordance with the present invention, examples being so-called quasi-single-field scanning, in which all the phase-displaced signal components originate from only a few graduation periods within the scanned scale region, or so-called Vernier scanning arrangements with different graduation periods of the structures provided on the scale and the scanner, etc.
Another advantage of the embodiment according to the present invention that can be listed is that now even if the scale should tilt in the plane of the scale, the phase relationship between the reference pulse signal generated and the incremental signals is preserved. This can be ascribed to the fact both the incremental signals and the reference pulse signal result from the same scanned point on the scale.
Because it is possible to eliminate the periodic incremental signal component in the reference pulse signal, an increased reliability of detection for the latter signal is also obtained. To that end, the most various options are disclosed below.
The provisions according to the present invention can also be realized in the most various optical position measuring devices. These include both systems that provide a collimating optical system in the light beam path and position measuring devices with so-called divergent illumination, that is, systems in which no collimating optics are provided in the light beam path, and so forth.
Furthermore, it is understood that both linear and rotational position measuring devices, as well as systems operated in incident light or transmitted light, can be embodied according to the present invention equally well.
Further advantages and details of the scanning unit of the present invention will become apparent from the ensuing description of a plurality of exemplary embodiments taken in conjunction with the accompanying drawings.
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